Macrophages, in conjunction with antigen or mitogen, stimulate T cells to produce a variety of immunoenhancing factors which serve to augment the immune responses of B cells, T cells, and macrophages. We have utilized phorbol myristic acetate-stimulated EL-4 mouse thymoma culture supernatants as the source of a variety of these mediators. A new factor, B cell growth factor (BCGF), distinct from a variety of other lymphokines has been described and shown to have a monomeric molecular weight of 12,000 and two isoelectric points of pH 6.4 and 7.4. The BCGF functions in synergy with Interleukin 2 (IL 2) and T cell replacing factor to enhance the antigen-specific antibody response. Additionally, we have characterized two additional EL-4-derived soluble mediators which exhibit the capacity to activate macrophages to become tumoricidal. One of these factors co-purifies with gamma Interferon (IFN-Gamma) whereas the second has a lower molecular weight and is clearly distinct from IFN-Gamma. Experiments have continued on the regulation of IFN-Gamma production by the development of a cloned T cell line which responds to PMA and IL 2 to produce very high titers of IFN-Gamma.